1. Field of the Invention
The present invention relates to a regenerative braking system for controlling a current flowing through the armature of an electric motor thereby to control a regenerative torque generated in the electric motor upon regenerative braking of a hybrid vehicle.
2. Description of the Related Art
Hybrid vehicles have an internal combustion engine and an electric motor for assisting in the output drive power of the internal combustion engine as propulsive power units for the vehicles. When such a hybrid vehicle is accelerated, the drive power of the internal combustion engine and also the drive power of the electric motor are applied to the drive wheels of the hybrid vehicle. In this manner, the output drive power of the internal combustion engine is reduced, the consumption of the fuel by the engine is lowered, and the exhaust performance of the engine is increased while at the same time the vehicle achieves a necessary acceleration capability.
The electric motor also operates as an electric generator. When the hybrid vehicle is decelerated, the kinetic energy of the vehicle is imparted from the drive wheels to the electric motor, which then operates as the electric generator. When the electric generator generates electric energy, it produces a regenerative torque which is applied to brake the vehicle (regenerative braking). The generated electric energy is retrieved to charge a power supply of the electric motor, e.g., a battery, an electric double-layer capacitor, or the like for effective energy utilization.
The regenerative torque generated in the electric motor is generally controlled by detecting a current flowing through the armature of the electric motor (hereinafter referred to as an "armature current"), and controlling the armature current in a feedback control loop so that the detected armature current will be equalized to a target current according to a given torque command. Specifically, a PWM (Pulse Width Modulation) control process is carried out to vary the pulse duration of a generated voltage outputted from the armature depending on the difference between the detected armature current and the target current for thereby regulating the armature current. With the armature current thus regulated to control the regenerative torque generated in the electric motor, if variations of the power supply voltage are relatively small as when a battery is used as the power supply of the electric motor, then since the ability of the actual current to follow the target current is constant, the regenerative torque generated in the electric motor can well be controlled. However, when a power supply whose output voltage is largely variable, such as an electric double-layer capacitor, is used as the power supply of the electric motor, the controllability of the regenerative torque generated in the electric motor is reduced as the ability of the armature current flowing from the armature to the power supply changes due to variations in the power supply voltage.
The above drawback may be eliminated by a software-implemented servo control process for inputting a detected power supply voltage, a detected armature current, a detected rotational speed of the electric motor, etc. as control parameters, and controlling the armature current depending on these control parameters.
In order to carry out the software-implemented servo control process, however, it is necessary to hold a large amount of data in advance for determining manipulative quantities to control an armature current from the values of the various control parameters, and also to perform complicated calculations for determining the manipulative quantities based on the large amount of data thus held. The complicated calculations need to be carried out at high speed so as not to cause a delay in the control of the electric motor. Consequently, the software-implemented servo control process requires, for its execution, a high-performance CPU and DSP capable of high-speed complex calculations. As a result, the system for effecting the regenerative braking is complex in structure and high in cost.